ABSTRACT: Methane is an important energy resource and significant long-lived greenhouse gas. Carbon and hydrogen isotope ratios have been used to better constrain the sources of methane but interpretations based on these two parameters alone can often be inconclusive. The precise measurement of a doubly substituted methane isotopologue, 13CH3D, is expected to add a critical new dimension to source signatures by providing the apparent temperature at which methane was formed or thermally equilibrated. We have developed a new method to precisely determine the relative abundance of 13CH3D by using tunable infrared laser direct absorption spectroscopy (TILDAS). The TILDAS instrument houses two continuous wave quantum cascade lasers; one tuned at 8.6 μm to measure 13CH3D, 12CH3D, and 12CH4, and the other at 7.5 μm to measure 13CH4. With the use of an astigmatic Herriott cell with an effective path length of 76 m, a precision of 0.2‰ (2σ) was achieved for the measurement of 13CH3D abundance in ca. 10 mL STP (i.e., 0.42 mmol) pure methane samples. Smaller quantity samples (ca. 0.5 mL STP) can be measured at lower precision. The accuracy of the Δ13CH3D measurement is 0.7‰ (2σ), evaluated by thermally equilibrating methane with a range of δD values. The precision of ±0.2‰ corresponds to uncertainties of ±7 °C at 25 °C and ±20 °C at 200 °C for estimates of apparent equilibrium temperatures. The TILDAS instrument offers a simple and precise method to determine 13CH3D in natural methane samples to distinguish geological and biological sources of methane in the atmosphere, hydrosphere, and lithosphere.